1. Field of the Invention
The present invention relates to a motor control system and method thereof, especially to a motor control system and method thereof for reducing the steady-state phase error and the transition time.
2. Description of the Prior Art
Please refer to FIG. 1(a), FIG. 1(b), and FIG. 1(c). FIG. 1(a) is a schematic diagram of a motor system 10 of the prior art. FIG. 1(b) is a schematic diagram of a motor controller 30 of the prior art. FIG. 1(c) is a schematic diagram of a transfer function of the motor controller 30 of the prior art. The motor system 10 of the prior art usually comprises a motor device 12, a motor controller 30, a pulse width modulation (PWM) generator 26, and a resistance-capacitance (RC) filter 28. The motor controller 30 is used for controlling the motor device 12. As shown in FIG. 1(b), the motor controller 30 comprises a frequency detector 11 and a phase detector 13. The frequency detector 11 is used for detecting the frequency error between a first feedback signal 44 and a reference clock signal 42. The phase detector 13 is used for detecting the phase error between the first feedback signal 44 and the reference clock signal 42. Furthermore, the motor controller 30 utilizes the frequency gain (Gf), the phase gain (Gp), and the sum gain (Gs) to adjust the frequency detector 11, the phase detector 13, and the sum result thereof, so as to determine the motor control output 31 and control the motor device 12 to achieve ideal predetermined frequency and phase. The schematic diagram of the transfer function of the motor controller 30 is shown in FIG. 1(c), i.e.
            G      s        ·          (                                    S            ·                          K              FD                        ·                          G              f                                +                                    K              PD                        ·                          G              p                                      S            )        ,wherein S is a Laplace variable of the signal frequency, KFD and KPD represent the gain of the frequency detector 11 and the phase detector 13 respectively, and Gf, Gp, and Gs are the programmable gain for adjusting the frequency detector 11, the phase detector 13, and the sum result thereof respectively. Although the motor system 10 has accurate frequency in steady state, there are still errors in the phase. The errors in the phase will affect the control manner of a device utilizing the motor system 10. For example, if the device utilizing the motor system 10 is an optical information recording/reproducing system, e.g. a CD-ROM drive, the CD-ROM drive will be affected by different control manners of the motor system, e.g. constant angular velocity (CAV) or constant linear velocity (CLV).
Please refer to FIG. 2(a) to FIG. 2(d). FIG. 2(a) is a schematic diagram of the radius (r) and the angular speed (rpm) of the CAV motor. FIG. 2(b) is a schematic diagram of the radius (r) and the angular speed (rpm) of the CLV motor. FIG. 2(c) is a schematic diagram of the radius (r) and the control voltage (Vm) of the CAV motor. FIG. 2(d) is a schematic diagram of the radius (r) and the control voltage (Vm) of the CLV motor. Under the CAV mode, the phase error is fixed because the inner and the outer angular speeds of the disc are the same even though the radii are different, so that the control of the motor is not being affected a lot. However, under the CLV mode, the phase error is different because the inner and the outer angular speeds of the disc are different by different radii, so that Gp and Gs should be adjusted to achieve better quality control. Moreover, before the motor system 10 of the prior art reaches the predetermined ideal angular speed and phase, it needs a period of time for the angular speed and the phase of the motor device 12 to reach the predetermined ideal value. This period of time is called transition time.
The motor controller 30 in the motor system 10 of the prior art needs a longer transition time for the frequency and the phase of the motor device 12 to converge to the predetermined ideal value. Moreover, even if the motor device 12 reaches the steady state and has the accurate frequency, there are usually still errors in the phase. In devices which request more accurate phase, this kind of the motor device 12 does not conform to the standard and causes problems in some devices. According to the above description, the motor system 10 of the prior art in steady state has some problems, e.g. larger phase error and longer transition time, to be improved.